The present invention relates to an anisotropic composite aerial. It is used in telecommunication applications, notably in the frequency band moving from about 50 MHz to about 4 GHz. The aerial of the invention can be used not only in emission but also in reception.
Aerials referred to as xe2x80x9cskinxe2x80x9d antenna are usually made up of a metal casing above which is arranged an element capable of radiating or receiving an electromagnetic field. The length of this element is generally in the vicinity of the half wavelength of the field to emit or to receive. It can be constituted by a slot drilled in a metal plate or of a metallic pattern (wire or strip).
FIG. 1 attached thus shows an aerial with an element 10 capable of radiating or receiving, a flat conductive plane 12, cylindrical or cubical conductive walls 13, a dielectric film 14 placed on the front side of the unit and serving as protection, and lastly a lead 16 connecting the element 10 to emission or reception means not shown. The electromagnetic field, radiated or received, is symbolically shown by the arrows R.
This type of aerial imposes severe restrictions on the distance D to be arranged between the radiating element and the conductive plane making up the bottom of the casing. This distance must be sufficiently large so that there is no destructive interference between the incident wave and the wave reflected by the casing, without however being excessive which would be harmful to the gain and to the bandwidth of the aerial.
In order to attempt to reduce these restrictions, it has been suggested adding a high-index dielectric between the element capable of radiating or receiving and the conductive plane, which allows decreasing the interval D. But this decrease is carried out to the detriment of the bandwidth of the aerial.
It has also been suggested using magnetic substrates in ferrite to tune the aerial on a certain frequency band. But the specific nature of this material (usually ceramic), as well as its mass and radio-electric properties restrict its use, in particular for large surfaces. Another considerable restriction is linked to the demagnetizing field of a substrate in ferrite. In fact, demagnetizing factors are associated with a cubic ferrite substrate, notably different from zero. This results in a dynamic demagnetizing field which is the product of a demagnetizing factor through the saturation magnetization of the ferrite. This field increases the resonance frequency while at the same time decreasing permeability of the ferrite substrate.
The static demagnetizing field (equal to the product of the demagnetizing factor in the direction of the field applied by the saturation magnetization),reduces the advantage of the ferrite substrate in the case that an outside magnetic field is applied in order to tune the properties of the aerial substrate. In fact, the field to be applied to the substrate is equal to the sum of the internal field and the demagnetizing field, and to increase the value of the field to be applied means increasing the strength of the system of magnets, or the consumption of an electromagnet.
The present invention has precisely the aim of resolving these disadvantages.
With this in mind, the invention advocates adding, between the conductive plane and the element capable of radiating or receiving, an anisotropic composite formed by a stack of alternate ferromagnetic and electrically insulated layers. These layers or film are perpendicular to the conductive plane. While they rest directly on this surface, they rest on their edge. Furthermore, these layers are directed or configured to be perpendicular (or approximately perpendicular) to the electrical component of the radiated or received field, component taken in the aerial plane.
The composite used in the invention is in itself recognized and sometimes called xe2x80x9cLIFTxe2x80x9d for Lamellar Insulator Ferromagnetic Tranche. This is described in the document FR-A-2 698 479. A measurement process of its electromagnetic characteristics is described in FR-A-2 699 683. Such a composite presents a high permeability and a low permittivity in the range of microwave frequency, for a plane wave arriving under normal incidence, with a linear polarization (magnetic field parallel to the layers and electric field perpendicular to the layers). It is possible to adjust the response in frequency of these materials by combining several ferromagnetic materials.
The composite in question is anisotropic, that is to say its electromagnetic properties are very different depending on the orientation of magnetic and electric fields in relation to the layers. If the electric field is perpendicular to the ferromagnetic layers, the material lets the electromagnetic wave penetrate. If, on the contrary, the electric field is parallel to the conductive lamina wafers, it is totally reflected by the material which then behaves like a metal.
When such an anisotropic composite is arranged in an aerial directly on the conductive plane, the surface impedance that it shows corresponds to a short-circuit seen through the line formed by the composite and for the favorable polarization (magnetic field parallel to the lamina wafers and electric field perpendicular to the layers). This impedance Z is defined by:
Zxe2x88x92Z0th(j.2xcfx80.N.e/xcex) 
where e is the composite thickness, Z0 a typical impedance, N2=xcex5xe2x8axa5. xcexc// and Z2=(xcexc//xcex5xe2x8axa5) where xcex5xe2x8axa5 and xcexc// are respectively the permittivity counted perpendicular to the layers and xcexc// the permeability counted parallel to the layers.
For other polarization, the impedance of the composite is near to that of a metal, that is to say near to zero.
The materials making up an anisotropic composite are light and easy to shape. Moreover, one can easily obtain responses in specific frequencies by taking advantage of the permeability of materials. In other respects, the conductive character of the composite for a particular direction of the field can be an advantage.
Moreover, the application on the anisotropic component of a magnetic field does not have the disadvantages encountered with ferrites. In fact, one can obtain high permeabilities with low volume fractions of magnetic matter. The demagnetizing field is thus proportional to the saturation magnetization divided by its volume fraction. One thus obtains values of static and dynamic demagnetizing field very much lower than in the case of ferrites. On an anisotropic composite aerial in compliance with the invention one can therefore use an external magnetic field, either to modify the tuning in frequency, or to adjust the level of permeability (by means of permanent magnets) to the desired frequency. In particular, an external magnetic field can be of use in reducing the magnetic losses to the working frequency.
In a precise manner, it is a general object of the present invention therefore to provide an aerial comprising an element capable of radiating or receiving an electromagnetic field, this element being arranged in front of a conductive plane, this aerial being typified in that it comprises moreover, between the element capable of radiating or receiving and the conductive plane, an anisotropic composite formed by a stack of alternate ferromagnetic and electrically insulated layers, these layers or film being perpendicular to the conductive plane and to the electrical component of the field radiated or picked up by the aerial.
The composite can be placed directly but not necessarily on the conductive plane.
As far as the element capable of radiating or receiving is concerned, it can be of any known shape straight or spiraled slot, straight or spiraled conductor wires or strips. The layers of composite must consequently always be oriented perpendicular (or approximately perpendicular) to the electrical component of the radiated or received field. This component is the component in the aerial plane (one does not take into account the component of the electric field oriented perpendicular to the plane of the aerial).